How do I fix mirror errors for Mirror Shape Key, without splitting and mirroring?
This error occurs when the mirrored mesh is not perfectly mirrored on the opposite side. The solution is to make a perfect mirror of both sides. But the mesh needs to have to be almost exactly the same on both sides also it must have even amounts of mesh on the mirrored side.

In edit mode select the half the vertex that needs to be mirrored.

Make sure the symmetry is on for the side you want to mirror.

Press G then Enter